Purification of isopropanol



March 11, 1952 s. w. WILSON 2,588,446

PURIFICATION OF ISOPROPANOL Filed June 9, 1949 ETHER+ IPOH ETHER+ LDWI BOILING' IMP L .B. IMF? CE U05 -IPOH WATER 4- HIGH BO/LEQS' HIGH-OILERS Patented Mar. 11, 1952 PURIFICATION OF ISOPROPANOL Samuel W; Wilson, Baton Rougc,.La., assignor to Standard Oil Development Company, a corporationofL-Delaware Application June 9, 1949-, Serial No. 97,962-

6 Claims.

thepresence of any one or combination of'chem ical. compounds, it can be said with reasonable assurance that; the odor of. crude-alcohol depends to a large extent on. the quality of the olefin stream employed in the olefin hydration operation... L'ikewisathe odorof a refined alcohol depends toa large extent onpthe quality'of'thecrude alcohol from. which it is prepared.

Olefin hydrocarbons, such as those produced by the crackingof'mineral oils, contain variable amounts of compounds having an obnoxious odor particularlysulfur compounds such as hydrogen sulfide, alkyl sulfides and mercaptans. These materials present, even in minute amounts, in the olefin stream to the acid catalyzed hydration process are believed to contribute to the obnoxiousodor, ofjthe crude alcohol since, whilesulfur alone hasno. smell, it is. clear-that-in combination with other elementsit is a powerful odiferous agent. The bad odor of alcohols has also attributed to the presence of so-called high-boiling polymer products formed in side reactions during the alcohol process. The odor of the polymer products is'strengthened. by the presence of any sulfur. compounds dissolvedtherein, although the odor. of some of the pure polymers themselves is by. no. means pleasing to the olfactory sense. A typical analysisofa sample of the so-called polymer product, in this case the so-called propyl oil, resulting from the production of isopropyl alcohol by thesulfuric, acid catalyzed hydration of propylene is as follows:

55" wt. percent sec.-heptano1 (B. 15137-140? C.)

21' wt. percentsem-octanol (B. P. 160-.165 'C.)

2 wt. per cent C1 ketone (B. P; 131 C.)

'13 wt. percent hydrocarbons (B. P. above 180 9 -wt-. per. cent. ethers (B. P. above 160 C.)

Tracesofsulfur compounds The composition of the propyl oil or so-ca lled 5 centration of dilute'crud'e isopropanol varies, according to the point from which the alcohol containing it is withdrawn in the concentrating tower, and the odor'likewise varies. Cuts can be identifiedv with anodor of camphor, of menthol, etc; Ithasal'so been reported that the presence of nitrogen compounds also contributes to the odor of alcohols.

The odorimparted' to-isopropyl alcohol by the propyl oil impurities is a hydrocarbon type odor. This-type of odor has been found to be effectively removed by a number of methods such asby careful and repeated conventional fractionations but particularly by the water extractive distillation method as described and claimed in U. S.,patent application Serial No. 24,626, filed May 1, 1948, and assignedv to the present assignee. However, there is a second type'of odor'which isfugitive in nature andwhich-maybesti be characterized as a mercaptan or thioaldehyde odor, and which, it is believed, is attributed to the presence of low boiling mercaptans or thioaldehydes which appear to be: decomposition" products of higher boiling impurities which break up'under alcohol purification distillation conditions. It has been found that this second. type of odor (hereinafter referred' to as mercaptan odor) is particularly noticeable when the alcohol production process is operated under upset generator conditions or when the purification process is operated under upset concentrator-conditions. When the-capacity of the generator: or'concentrator is exceeded some isopropyl alcohol creeps down into thereboiler associated with" the generator or concentrator wherein ahigh temperature prevails and where in the case ofthe generator a particular concentration of sulfuric acid exists. Under theseconditions it is felt that the heat and/or the sulfuric acid acts-as a. reducing agent-on the isopropyl alcohol which decomposes into lower boiling impurities, particularly mercaptan, thioether, and thioaldehyde compounds which are volatile. These impurities go overhead with the alcohol from the generator. Subsequently in the concentrator they are not all removed with the low boiling overhead, but remain in traces of parts per'million in the isopropyl alcohol product which is-removed as a top sidestream from the concentrator. Likewise if the concentrator conditions: are upset thesame decomposition occurs and the problem becomes aggravated. It has also been:theorized-that the second type of odor is due to the-decomposition of sulfurized esters whichareformed in minute amounts during the propylene: absorption in sulfuric acid. These ever, the nature of the chemical reactions tending to bind these malodorous compounds is such that unstable addition compounds result and the malodorous compounds are liberated and volatilized to some extent during all alcohol finishing operations of the types described. For instance,

in typical alcohol finishing operations the following problems are encountered. Crude alcohol, after passing through the heads operation to remove ether and other low boilers, is fed to a finishing column in which certain low boiling materials are taken overhead with a purified alcohol taken as a sidestream. Materials contributing Zillto mercaptan odor appear to be lower boiling than the alcohol proper but are nevertheless not completely removed from the alcohol sidestream. This result is believed to be due in large part to the fact that decomposition reactions liberating low boiling malodorous compounds occur during the distillation, allowing these malodorous compounds to pass overhead. Similar diificulties are encountered when applying water extractive distillation to the same crude alcohol. In fact, water extractive distillation appears to concentrate the compounds giving rise to mercaptan odor along with the alcohol so that, in the final alcohol finishing operation, an alcohol product high in mercaotan odor is obtained. This seems to be particularly true of isopropyl alcohol prepared by the so-called weak acid method as described below.

It has been found that the quality of refined isopropanol can be improved bya process called reverse distillation" performed upon the crude aqueous isopropanol particularly when the crude isopropanol is kept at a pH on the basic side.

In the normal finishing of crude aqueous isopropanol the crude alcohol is distilled in a first column called the heads column, where ether and. other low-boiling impurities are separated overhead. The dilute alcohol is then led to a concentrating column where 91% isopropanol in the form of a water azeotrope is removed overhead from water and the high boiling impurities. In the reverse distillation operation the crude alcohol is distilled in a first column from second column, which now acts as a heads column, in which the ether and low-boiling impurities are taken overhead from the alcohol-H2O azeotrope. Any remaining high-boilers in the alcohol may be removed in a third column if necessary.

Crude isopropyl alcohol may be produced by the sulfuric acid catalyzed hydration of propylene by the weak acid method or the strong acid method. In the former process a relatively rich propylene stream containing about 50-80 vo u e per cent propylene is absorbed in sulfuric acid of approximately 60 to 80 weight per cent concentrations, preferably '70 weight per cent, at about 170 F. and approximately 250 p. s. 1-. g. pressure to form an extract comprising isopropyl sulfate which is partially hydrolyzed to alcohol during the absorption. The hydrolysis is completed by dilution of the extract with water to approximately 45 weight per cent acid strength and maintaining the extract at a temperature of about 190 F. for a period of about 10 minutes residence time. The hydrolysis products are then distilled in an alcohol generator wherein crude alcohol vapors are removed overhead and condensed, and wherein spent sulfuric acid is recovered as bottoms for reconcentration and subsequent re-use in the absorption process. The crude isopropyl alcohol contains approximately 30-60% by volume of isopropyl alcohol, some impurities including hydrocarbons, isopropyl ether,

acetone and unknown impurities, and the balance water. The crude alcohol is condensed and cooled and is ready for the purification stage of the process.

In the strong acid process a propylene stream of relatively lower propylene content, i. e., about 30-60 volume per cent propylene is contacted under similar conditions with a stronger sulfuric acid, e. g., acid of -95 weight per cent concentration, preferably -88 weight per cent, whereby an extract is formed as above. The extract goes through a similar hydrolysis and stripping treatment to produce a crude isopropyl alcohol as described for the weak-acid process.

When the isopropyl alcohol is purified by conventional fractionation it is led to a heads column the purpose of which is to remove ether and other low-boiling water-insoluble impurities, such as hydrocarbons, hydrocarbon polymers, oxygenated compounds, and unknown impurities. This is accomplished by a steam stripping operation whereby the lighter materials are withdrawn overhead while weak aqueous alcohol is recovered as bottoms. The weak aqueous alcohol is then distilled in a concentrating column where the alcohol is concentrated to a composition approaching its water azeotrope, viz., 91 volume per cent isopropyl alcohol-9 volume per cent water. The alcohol of azeotropic composition may be further distilled for removal of acetone if present, or concentrated further to 95 volume per cent or 99 volume per cent isopropyl alcohol if desired.

The process of this invention will be more clearly understood from the attached drawing which represents a semidiagrammatic elevational sketch of one type of apparatus for carrying out the process.

Referring to the drawing numeral l represents a distillation tower to which crude malodorous aqueous isopropanol of the type previously described is introduced via line 2. The pH of the crude alcohol is preferably adjusted to the basic side before the distillation is initiated. This may be done by any of the conventional means such as by addition of caustic soda, caustic potash, the Water-soluble alkali and alkaline earth carbonates, basic salts, organic bases such as the amines, etc, by addition of ammonia, etc. The pH may be adjusted by treatment of the crude alcohol feed before it enters the tower] or it may be adjusted thereafter and controlled at the proper pH by addition of caustic to the tower or via the reflux line H, etc. This invention is not limited by any manner in which the pH is controlled provided it is always kept on the basic side. Heat is applied to the tower by conventional means such as open steam but preferably by means of a reboiler (not illustrated). Suflicient; ly high temperature is maintained to take over head via line 3 isopropyl ether, low-boiling impurities such as acetone, hydrocarbons, hydrocarbon polymers, etc. and the isopropanol in the form of its water azeotrope. The bottoms removed via line 4 comprise water and the highboiling impurities. Part of the overhead may be condensed and refluxed to the tower l via line I I. The overhead from tower l, with or without prior condensation, is then conducted to distillation column 5 which serves as a heads column to remove overhead via line 6 the ether and other low-boiling impurities, including malodorous bodies, leaving as bottoms the purified isopropanol-Water azeotrope which is removed via line 1 to storage. Part of the overhead from tower 5 may be condensed and refluxed to column 5 via line 12. If the alcohol bottoms from column 5 still contain some high-boiling impurities the alcohol is led to column 8 wherein the alcohol is distilled overhead as the azeotrope via line 9. Conventional reflux may be made of the condensed distillate via line 13. High-boilers are removed as bottoms via line Ill.

The following runs demonstrate the efiectiveness of the reverse distillation procedure on the basic crude alcohol for the removal of mercaptan odor. The distillations represent the results of heads column runs carried out on a number of successive alcohol overhead cuts obtained by reverse distillation of crude isopropanol as described in the above process. The heads column was operated at a 5: 1 reflux ratio. The pH of the alcohol was adjusted with caustic soda.

propanol overhead from the high-boiling impurities and water, conducting the distillate from the first distillation zone to a second distillation zone, removing low-boiling impurities including malodorous bodies giving rise to mercaptan odor overhead from the second distillation zone and recovering highly refined isopropanol free of malodorous bodies as bottoms from the second distillation zone.

2. A process according to claim 1 in which the pl-I of the crude alcohol is adjusted to the basic side by the addition of caustic soda.

3. A process according to claim 1 in which the pH is maintained in the range of 7.5 to 9.5.

l. A process for the production of highly refined isopropanol from crude aqueous isopropanol prepared by the sulfuric acid-catalyzed hydration of propylene, said crude alcohol containing lowboiling and high-boiling impurities including malodorous bodies giving rise to mercaptan odor which comprises, adjusting the pH of the crude alcohol to the basic side, distilling the basic crude alcohol in a first distillation zone to remove the low-boiling impurities and isopropanol overhead from the high-boiling impurities and water, conducting the distillate from the first distillation zone to a second distillation zone, removing low-boiling impurities including malodorous bodies overhead from the second distillation zone and recovering highly refined isopropanol free of malodorous bodies as bottoms from the second distillation zone.

5. A process according to claim 4 in which the 1 MO=mercaptan odor, graded from 1 to 4; the higher the numeral the greater the mercaptau odor.

2 NMO=no merceptan odor.

The data illustrate that mercaptan odor is substantially eliminated by the reverse distillation of the crude particularly at basic pHs. Although some improvement in mercaptan odor is obtained when the process of reverse distillation is applied to a crude alcohol which contains acid as evidenced by the reduction in mercaptan odor," complete removal of mercaptan odor is not obtained. It is therefore preferred to maintain the crude isopropanol at a basic pH preferably in the, neighborhood of 7.5 to 9.5.

What is claimed is:

1. A process for the production of highly refined isopropanol of improved odor quality from crude aqueous isopropanol prepared by the sulfuric acid-catalyzed hydration of propylene and containing low-boiling and high-boiling impurities including malodorous bodies giving rise to mercaptan odor which comprises adjusting the pH of the crude alcohol to the basic side, distilling the basic crude alcohol in a first distillation zone to remove the low-boiling impurities and isopH of the crude alcohol is adjusted to the basic side by the addition of caustic soda.

6. A process according to claim 4 in which the pH is maintained in the range of 7.5 to 9.5.

SAMUEL W. WILSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,600,437 Taveau Sept. 21, 1926 1,614,877 Clapp Jan. 18, 1927 1,702,495 Clapp Feb. 19, 1929 1,933,505 Merley Oct. 31, 1933 1,995,597 Archibald et a1 Mar. 26, 1935 2,080,064 Roelfsemg May 11, 1937 2,080,111 Bump May 11, 1937 2,470,222 Patterson May 17, 1949 

1. A PROCESS FOR THE PRODUCTION OF HIGH REFINED ISOPROPANOL OF IMPROVED ODOR QUALITY FROM CRUDE AQUEOUS ISOPROPANOL PREPARED BY THE SULFURIC ACID-CATALYZED HYDRATION OF PROPYLENE AND CONTAINING LOW-BOILING AND HIGH-BOILING IMPURITIES INCLUDING MALODOROUS BODIES GIVING RISE TO "MERCAPTAN" ODOR WHICH COMPRISES ADJUSTING THE PH OF THE CRUDE ALCOHOL TO THE BASIC SIDE, DISTILLING THE BASIC CRUDE ALCOHOL IN A FIRST DISTILLATION ZONE TO REMOVE THE LOWS-BOILING IMPURITIES AND ISOPROPANOL OVERHEAD FROM THE HIGH-BOILING IMPURITTIES AND WATER, CONDUCTING THE DISTILLATE FROM THE FIRST DISTILLATION ZONE TO A SECOND DISTILLATION ZONE, REMVOING LOW-BOILING IMPURITIES INCLUDING MALODOROUS BODIES GIVING RISE TO "MERCAPTAN" ODOR OVERHEAD FROM THE SECOND DISTILLATION ZONE AND RECOVERING HIGHLY REFINED ISOPROPANOL FREE OF MALODOROUS BODIES AS BOTTOMS FROM THE SECOND DISTILLATION ZONE. 